1. Technical Field
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that uses a fixing belt for fixing a toner image, and an image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features, incorporating such a fixing device.
2. Background Art
Fixing devices are employed in electrophotographic image forming apparatuses, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features, wherein an image formed of toner particles is fixed in place with heat and pressure on a recording medium such as a sheet of paper.
Various types of fixing devices are known in the art. One particular type is a belt-based fixing device employing a rotatable, endless belt that can be heated rapidly and efficiently to a desired operational temperature, which allows for processing a toner image with an extremely short warm-up time and first-print time without causing image defects even at high processing speeds.
For example, one belt-based fixing device has been proposed, including a rotatable, endless fuser belt looped into a generally cylindrical configuration, a stationary pad disposed inside the loop of the belt, and a pressure roller pressing against the stationary pad via the belt to form a fixing nip therebetween. Also included are a tubular belt holder of thermally conductive metal, or heat pipe, disposed inside the loop of the belt to face the inner circumferential surface of the belt except at the fixing nip, a heater disposed inside the heat pipe to radiate heat to the heat pipe, and a reinforcing plate disposed in contact with the stationary pad inside the heat pipe to reinforce the fuser pad.
During operation, the heater radiates heat to the heat pipe, from which heat is imparted to the entire circumference of the fuser belt entrained around the heat pipe. The recording sheet is conveyed through the fixing nip, at which the toner image is fixed in place with heat from the fuser belt melting and fusing toner particles, and pressure between the fuser pad and the pressure roller causing molten toner to set onto the recording sheet.
Another, similar fixing device has also been proposed, which employs a heat shield interposed between the belt and the heater to intercept transmission of heat from the heater to the belt, thereby preventing excessive heating of those portions of the belt which do not contact the recording medium during passage through the fixing nip.
The inventors have recognized that one problem associated with the belt-based fixing device is inefficient, non-uniform heating of the belt, which has its inboard portion (i.e., that portion around the longitudinal center of the belt adapted to contact the recording medium during passage through the fixing nip) and its outboard portion (i.e., that portion around the longitudinal end of the belt adapted to remain away from the recording medium during passage through the fixing nip) subjected to different amounts of heat upon activation of the fixing device.
For example, the outboard portion of the belt can be excessively heated after startup of the fixing device, for example, where the belt is warmed stably and sufficiently to a desired operational temperature during sequential processing of multiple recording media. Moreover, using a heat shield to prevent overheating of the outboard portion of the belt can in turn cause heat to escape from the laterally outward, peripheral part of the inboard portion to the outboard portion of the belt during startup of the fixing device, for example, initially in the morning, where the belt has been cooled to an ambient temperature due to an extended period of deactivation.
Those problems, if not properly addressed, would cause various adverse consequences, which are particularly pronounced in the fast, belt-based fixing device that can process a toner image with an extremely short warm-up time and first-print time.